Polymers may be classified as thermosoftening or .
Thermosoftening polymers are sometimes called thermoplastic.
Thermosoftening polymers include , ,
and . A polymer molecule is a long chain of carbon atoms held together by strong covalent bonds. The forces in between the molecules are relatively weak but parts of the chains can sometimes line up side by side to form tiny crystals.
These crystals can form links between the chains and hold the structure together. When the polymer is heated the crystals will melt, the material will become very soft and can flow slowly like a thick liquid. In this state the polymer can fill a mould and be cast into a shape. When the polymer cools down, new crystals can form between the chains and the new shape is fixed. The same polymer can be reheated and remoulded. Such polymers are called thermosoftening (meaning that they go soft when you heat them).
The picture below shows the tangled polymer chains which have lined up and crystallized in the pink region.
Some polymers do not form crystals. They soften when heated and harden when cooled down again. When cold, they are not crystalline but glassy. These polymers are also called thermosoftening.
Polymers may be classified as or thermosetting.
Thermosetting polymers have their chains cross linked by . The polymer is originally placed into a mould to form the desired shape. The polymer is then heated and chemical reactions occur to form the cross links between the chains. The resulting three dimensional solid structure cannot then be changed. Further heating will not cause the polymer to soften or change shape (unlike polymers). Examples of thermosetting polymers are
1. Melamine - used in furniture.
2. Bakelite - used for saucepan handles and electric light fittings.
3. Epoxy resins - used in many glues.
The picture below shows a typical structure for a thermosetting polymer. The red lines represent the cross links between the chains.
Disposing of Polymers.
Although polymers are very useful materials, there are problems with disposal of unwanted articles.Polymers are not biodegradable.
This means that bacteria will not break them down. If they are buried in the ground (landfill) they simply remain as polymers in the ground. Products from plant material (wood, paper, cotton etc.)
are biodegradable. When buried, bacteria and fungi break them down
into useful nutrients for further plant growth. Nature recycles its own products! Polymers produce materials (poisons) when burnt,
in addition to the expected products from combustion of a which are carbon dioxide, water, and (soot). Those which contain chlorine ( for example)
also produce on burning. Those which contain nitrogen (nylon for example) produce hydrogen cyanide. Polymers are a fire hazard. Many people die from the smoke of burning polymers in house fires, long before the fire reaches them. Burning polymers is not a good way of disposing of them.
Solutions.
More and more polymers are being recycled. This is not as cost effective as recycling metals, but we don't want to live amongst piles of (unrotting) plastic. Research into biodegradable polymers
will increasingly provide useful replacements for the main polymers of today.
